A dilemma exists in present day communications. As a result of the increased usage of cellular phones, there is a large amount of interference in bands around 900 MHz, that was not there previously. Consequently, many communications devices that used to work fine, before the advent of cellular phones are nonfunctional due to the added noise. Standard high frequency filters do not have the selectivity to filter out enough of this new noise to remedy the problem. Crystal filters have the selectivity to remedy the problem but operate at a much lower frequency.
The standard approach to filtering uses filters of the type available from TXRX, 8625 Industrial Parkway, Angola, N.Y., 14006, in their sales brochure, titled Multicouplers, Duplexers, Cavity Filters, 30 MHz to 960 MHz. Their Vari-Notch.RTM. and Series-Notch.RTM. filters are covered by U.S. Pat. Nos. 4,186,359 and 4,206,428. However, the filters of this brochure are inadequate for eliminating the noise from cellular phones. Those working in this area are baffled as to how to overcome the above mentioned dilemma.
Down converting a signal, filtering it and then up converting it and inputting it into a speaker was disclosed by U.S. Pat. No. 2,608,648. However, the input and output signal are typically different from each other in this invention.
Transponders at times have their input and output frequencies equal to one another. Heterodyning is a known technique used in some repeaters, transponders, carrier recovery systems, and phase stabilizing circuits. Some patents have issued having their input and output frequencies equal and/or that use heterodyning. The present inventor is aware of these U.S. Pat. Nos. 3,019,296, 3,946,293, 4,306,404, 4,743,843, and 4,972,346. U.S. Pat. No. 4,972,346 is of much significance to the present invention. The repeater of U.S. Pat. No. 4,783,843 is concerned with eliminating dead zones in the cellular phone bands. Thus, this repeater is set up to transpond two adjacent bands. The transponder U.S. Pat. No. 4,972,346 is concerned with eliminating the "singing" phenomenon. These problems are not the concerns of the present invention. U.S. Pat. No. 4,792,346 also as a prefilter/amplifier unit and a filter/amplifier unit after heterodyning in prior art FIG. 2, while U.S. Pat. No. 4,783,843 also has a filter before and after heterodyning. However, these patents do not discuss the purpose and the details of the filters and amplifiers before and after the heterodyning part of the circuit. These patents are for devices that are designed to connect to a known pair of antennas. The present inventor was the first to recognize the need for a device that could be inserted between a pre-existing antenna and receiver so that they can continue to operate with the additional noise from cellular phones. Furthermore, these heterodyning units all use a local oscillator of a fixed frequency, making them useful only in one frequency band.